The present invention relates to setting particulate plugs in horizontal well bores, and more particularly, in certain embodiments, to methods involving low-rate pumping of slurries.
In both vertical and horizontal well bores, it is frequently desirable to treat a subterranean formation at various locations of interest along the length of the well bore. In general, a well bore may penetrate various reservoirs, intervals, or other zones of interest. In some instances, the length or extent of an interval may make it impractical to apply a single treatment to the complete interval. When treating a reservoir from a well bore, especially from well bores that are deviated, horizontal, or inverted, it is difficult to control the creation of multi-zone fractures along the well bore without cementing a liner to the well bore and mechanically isolating the zone being treated from either previously treated zones or zones not yet treated.
At various points in treatment of a well bore, plugs may be useful, inter alia, to isolate a zone of interest. The creation of an interval zone with the use of one or more plugs may provide for distinct, sequential treatments of various zones of interest. Plugs may comprise valves, mechanical devices such as packers, and/or liquid or solid barriers, e.g., a plug made of particulates. Typically, particulate plugs have been created only in vertical well bores, due to difficulties encountered in creating particulate plugs in deviated, horizontal, or inverted well bores.
Generally, well bores may be cased or uncased through treatment zones. For example, a cased, vertical well bore may be perforated through a first, lower zone of interest. A pumping conduit may then be extended into the well bore to a depth above the first zone of interest, and a packer may be positioned to prevent the flow of fracturing fluid upwardly between the outside of the conduit and the inside of the casing. A fracturing fluid may then be injected into the vertical well bore to fracture the formation through the perforations or, in the situation of an uncased well bore, through a notched area of the formation of interest. After the fracturing is completed, a particulate plug may be positioned over the fractured formation by filling the well bore with particulates to a suitable level. Thereafter, a formation above the particulate plug may be perforated and fractured by the same technique. By the use of particulate plugs of a variety of depths, a plurality of formations in a vertical well bore may be fractured independently of one another. Typically, each zone is perforated separately so that the particulate plug effectively isolates all the zones below the zone being treated. Zones above the zone being treated are typically perforated subsequently or are isolated from the zone being treated by the packer.
In horizontal well bores, by contrast, particulate plugs typically have not been readily usable. In some instances, a particulate plug may generally slump and expose the perforations and/or fractures in a previously treated zone to the fluid pressure imposed to treat a location uphole from the previously treated zone.
Typically, pairs of packers or other mechanical isolation devices have been used to isolate treatment zones in a section of a well bore that is deviated, horizontal, or inverted. The packers may be carried into the well on a tubing or other suitable work string. The first packer may be set downhole of the treatment zone, and the second packer may be set uphole of the treatment zone. The treatment fluid may thereafter be placed into the treatment zone between the two packers to treat the horizontal well bore at the desired location. A plurality of zones in the horizontal well bore may be readily treated using this technique, but it is a relatively expensive and complicated technique. Alternatively, treatments of horizontal well bores have utilized traditional methods of gravel packing. As used herein, “gravel packing” refers to the pumping and placement of a quantity of desired particulates into the unconsolidated formation in an area adjacent the well bore. Such procedures may be time consuming and costly for formations with multiple treatment zones in horizontal sections of the well.
Setting particulate plugs in horizontal well bores is generally challenging. Traditional methods of setting particulate plugs in a vertical well bore may not be directly transferable to a horizontal well bore. For example, setting a particulate plug in a previous treatment zone of a horizontal well bore may require that the particulate plug have sufficient height to create a bridge across either a perforation or casing. However, a low concentration slurry—as is generally required to provide a pumpable slurry—may only partially fill a horizontal well bore due to gravity-induced settling. Moreover, if the well bore is cased, insufficient leak-off may hamper particulate deposition.
Previous attempts to set particulate plugs in horizontal well bores have been limited by the pumpable densities of the slurry and the resulting effective height of the particulate plug. For example, slurries with excessive densities may result in particulate deposits within the pumping conduit. Alternatively, low concentration slurries may not permit sufficient deposition of particulates within the well bore to form particulate plugs. Setting a particulate plug in a horizontal well bore, especially when utilizing low concentration slurries, often requires waiting for a certain degree of fracture closure to be able to bridge the particulate plug on the perforations. Indeed, attempts to form successful bridges have often failed, and those skilled in the art and practicing in the industry have typically engaged in practices which did not require the creation of bridges in horizontal well bores.
More recently, Halliburton Energy Services, Inc., of Duncan, Okla., has introduced and proven technologies for hydrajet treatment methods for both horizontal and vertical well bores. The methods may include the step of drilling a well bore into the subterranean formation of interest. Next, the well bore may or may not be cased and cemented, depending upon a number of factors, including the nature and structure of the subterranean formation. The casing and cement sheath, if installed, and well bore may then be perforated using a high-pressure fluid being ejected from a hydrajetting tool. A first zone of the subterranean formation may then be fractured and treated. Then, the first zone may temporarily be plugged or partially sealed by installing a viscous isolation fluid into the well bore adjacent to the one or more fractures and/or in the openings thereof, so that subsequent zones can be fractured and additional well operations can be performed. In one method, this process may generally be referred to by Halliburton as the CobraMax® H service, or stimulation method, and is described in U.S. Pat. No. 7,225,869, which is incorporated herein by reference. Such processes have been most successful in well bores that are deviated, horizontal, or inverted, where casing the hole is difficult and expensive. By using such techniques, it may be possible to generate one or more independent, single plane hydraulic fractures, and, therefore, a well bore that is deviated, horizontal, or inverted may be completed without the need to case the well bore. Furthermore, even when highly deviated or horizontal well bores are cased, hydrajetting the perforations and fractures in such well bores may generally result in a more effective fracturing method than using traditional explosive charge perforation and fracturing techniques. However, the isolation fluid may be expensive, environmentally hazardous, and pose operational logistics challenges. For example, it may be difficult to remove these materials in preparation for production. Therefore, an alternate method of providing zone isolation in horizontal well bores is desirable to enhance these processes and provide greater reliability.